This invention relates to an amplifier arrangement comprising a first and a second output transistor each having a base, a collector and an emitter, an input terminal, an output terminal, a first and a second supply terminal, and a bias stage for generating a bias voltage between the bases of the first and the second output transistors, the bases of the first and the second output transistors both being coupled to the input terminal, the collectors of the first and the second output transistors being coupled to the first and the second supply terminals respectively, the emitters of the first and the second output transistors being coupled to the output terminal, the bias voltage having a negative thermal response, and an element of the bias stage being thermally coupled to the first and the second output transistors
In the present document a negative thermal response is to be understood to mean a thermal response resulting from a negative temperature coefficient.
Such an amplifier arrangement, which can be constructed by means of discrete components or as an integrated circuit, can be used in general for driving a load coupled to the output terminal in dependence upon an input signal applied to the input terminal.
Such an amplifier arrangement is known, inter alia, from the pre-published Netherlands Patent Application NL 65 10 718. In the known amplifier arrangement the bias stage comprises a resistor, a voltage divider having a first terminal, a tap and a second terminal, and a transistor having a base coupled to the tap of the voltage divider, a collector coupled to the first terminal of the voltage divider and to the base of the first output transistor, and an emitter coupled to the second terminal of the voltage divider and to the base of the second output transistor. The first terminal of the voltage divider is coupled to the first supply terminal by means of the resistor and the second terminal of the voltage divider is coupled to the second supply terminal by means of an input transistor coupled to the input terminal. In the known amplifier arrangement the voltage divider and the transistor function as a so-called base-emitter voltage multiplier, the bias voltage generated by the bias stage being equal to the product of a ratio defined by the voltage divider and the base-emitter voltage of the transistor, and the thermally coupled element of the bias stage being formed by the transistor. By means of the bias voltage applied between the bases of the output transistors the output transistors can be operated in class AB in order to reduce the distortion of the output signal, for which operation the bias voltage in the known amplifier arrangement should be approximately two times the base-emitter voltage of the transistor. As a result of the thermal coupling between the element formed by the transistor and the respective output transistors, the known amplifier arrangement is protected against an undesired influence of a temperature rise on the class AB operation. When the output transistors become warmer owing to a temperature rise, a current defined by the bias voltage and flowing through the output transistors will increase as a result of a physical property inherent in the output transistors. However, when the element constituted by the transistor becomes warmer owing to the temperature rise and the thermal coupling, the bias voltage in the case of an equal current through the transistor will decrease as a result of a physical property inherent in the transistor. As a consequence, the increase of the current through the output transistors is counteracted by a decrease of the bias voltage (the negative thermal response), so that a substantially temperature-independent class AB operation can be obtained in the case of an appropriate dimensioning.
However, a disadvantage of the known amplifier arrangement is that for the substantially temperature-independent class AB operation a perfect thermal coupling is a prerequisite. If the perfect thermal coupling is not achieved the current in the output transistors will increase despite a decrease of the bias voltage (the negative thermal response), which increase may give rise to thermal instability of the amplifier arrangement.